Relentless Rats gets +1/+1 for each other creature on the battlefield named Relentless Rats.
A deck can have any number of cards named Relentless Rats.
The second ability of Relentless Rats lets you ignore the “four-of” rule. It doesn’t let you ignore format legality. For example, during a Masters 25 Limited event, you can’t add Relentless Rats from your personal collection.